JPH08104922A - Production of high strength steel pipe excellent in low temperature toughness - Google Patents

Abstract

PURPOSE: To produce a high strength steel pipe excellent in sour resistance, low temp. toughness and weldability by working a low alloy steel having a specified compsn. into a steel pipe and thereafter executing heat treatment by induction heating. CONSTITUTION: This steel has a compsn. contg., by weight, 0.02 to 0.09% C, <=0.6% Si, 1.3 to 2.0% Mn, <=0.015% P, <=0.010% S, 0.3 to 1.2% Ni, 0.9 to 1.2% Co, 0.1 to 0.5% Mo, 0.01 to 0.06% Nb 0.005 to 0.03% Ti, <=0.06% Al and 0.001 to 0.006% N, and the balance Fe with inevitable impurities. The steel is formed into a seamless steel pipe, an electric resistance welded pipe and a UOE steel pipe, and after that, induction heating is used from <=50 deg.C, and it is hardened under heating to the temp. range of the Ac3 point to 1000 deg.C and is subjected to tempering treatment in the temp. range of 450 deg.C to the Ac1 point. Thus, the ultrahigh strength line pipe of the API specification×100 or above can stably be produced.

Description

Detailed Description of the Invention

[0001]

This invention relates to the American Petroleum Institute (AP
I) High strength of X100 or more in the standard (about 689 in yield strength)
N / mm ² or more) and excellent low-temperature toughness, on-site weldability and sour resistance, and is applicable to the production of UOE steel pipes, seamless steel pipes, and electric resistance welded steel pipes.

[0002]

2. Description of the Related Art Line pipes used in pipelines for transporting crude oil and natural gas over long distances are (1) for improving transportation efficiency by increasing pressure and (2) for improving welding efficiency in the field by reducing wall thickness. There is a tendency for the tensile strength to become higher and higher. Until now, line pipes up to X80 according to the API standard have been put into practical use, but there has recently been a need for line pipes with higher strength.

At present, ultrahigh strength line pipes of X100 or more are manufactured by the method of manufacturing X80 class line pipe (NKK technique, N
o. 138 (1992), pp24-31 and The.
7th Offshore Mechanics a
nd Arctic Engineering (198
8), Volume V, pp179-185), but these line pipes have low temperature toughness, field weldability, joint softening, sour resistance (hydrogen induced cracking resistance, sulfide stress corrosion cracking resistance). However, there is a demand for early development of an epoch-making ultra-high-strength steel pipe (line pipe) that overcomes these problems.

[0004]

DISCLOSURE OF THE INVENTION The present invention is directed to low temperature toughness,
It is an object of the present invention to provide a method for producing an ultrahigh strength steel pipe having an X100 or higher that can simultaneously achieve various properties such as field weldability and sour resistance.

[0005]

The gist of the present invention is that, by weight%, C: 0.02 to 0.09%, Si:
0.6% or less, Mn: 1.3 to 2.0%, P: 0.01
5% or less, S: 0.0010% or less, Ni: 0.3 to
1.2%, Cu: 0.9 to 1.2%, Mo: 0.1
0.5%, Nb: 0.01 to 0.06%, Ti: 0.0
05-0.03%, Al: 0.06% or less, N: 0.0
01-0.006%, and if necessary, further C
a: 0.001 to 0.005%, V: 0.01 to 0.1
A steel pipe containing 1% or 2 or more of 0% and Cr: 0.1 to 0.5%, the balance being iron and unavoidable impurities and reheating, and rolling-molding the produced steel pipe, Of low temperature toughness characterized by heating by induction heating to a temperature range of Ac ₃ point to 1000 ° C. from a temperature of ≦ ° C. or lower to perform quenching treatment, and subsequently tempering in a temperature range of 450 ° C. to Ac ₁ point. It is an excellent method for manufacturing high-strength steel pipes.

The method of manufacturing the ultra high strength steel pipe of the present invention will be described in detail below. The features of the present invention are (1) 0.9
Low C- containing ~ 1.2% Cu and treated with extremely low S
A steel pipe manufactured by rolling forming a Ni-Cu-Mo-Nb-Ti steel is (2) heated to a certain temperature range by induction heating for quenching treatment, and then tempered in a certain temperature range. Where the ultra high strength and excellent low temperature toughness,
It achieves local weldability and sour resistance at the same time.

Conventionally, Cu-precipitated steel has been used for high-tensile steel for pressure vessels (tensile strength 784 N / mm ² grade) and the like, but no development example of ultrahigh-strength line pipe of X100 or more is found. This is because Cu precipitation hardened steel is easy to obtain strength, but low temperature toughness and sour resistance (especially resistance to hydrogen induced cracking (HIC), hereinafter referred to as HIC resistance) were insufficient as a line pipe. it is conceivable that.

First, although it has low temperature toughness, in a pipeline, the propagation stopping property is extremely important as well as the brittle fracture occurrence property. Although the conventional Cu precipitation hardening steel has a definite brittle fracture generation characteristic represented by Charpy characteristics,
The brittle fracture termination properties were not sufficient. This is due to (1) insufficient microstructure miniaturization, and (2) the separation that occurs on the fracture surface of the test piece, such as the so-called Charpy impact test, has not been used (separation occurs during the impact test. It is believed that the delamination phenomenon parallel to the plate surface improves the propagation stopping property of the brittle crack by reducing the triaxial stress level at the brittle crack tip).

Next, it can be mentioned that the HIC resistance was not sufficient. This is because the addition of 0.9 to 1.3% Cu suppresses the corrosion reaction on the steel surface and improves the HIC resistance, but as a result of not purifying the steel or treating with Ca, it is slightly It is considered that HIC is generated even if hydrogen invades. Generally, ultra high strength line pipes of X100 or more are used in a dry and sweet environment that does not contain hydrogen sulfide, but in some cases a small amount of hydrogen sulfide may be generated due to intrusion of seawater, etc. It is extremely important for the safety of strength linepipes.

First, the reasons for limiting the manufacturing conditions of the present invention will be described. After reheating, the steel pipe manufactured by rolling and forming is
Ac ₃ points to 1000 ° C by induction heating from a temperature of 00 ° C or less
And heat treatment at 450 ℃
c It is necessary to perform tempering within the temperature range of ₁ point. Steel pipe 5
Ac ₃ points to 1000 ° C by induction heating from a temperature of 00 ° C or less
The reason for heating to the temperature range of 1 is to utilize transformation strengthening due to formation of bainite structure by subsequent quenching treatment, and to secure solid solution Cu effective for precipitation hardening by sufficiently solutionizing Cu. is there. However, if the heating temperature exceeds 1000 ° C., austenite grains grow during reheating and the crystal grains become large, resulting in deterioration of low temperature toughness and sour resistance. Therefore, the upper limit of the reheating temperature is 1000 ° C.
And The lower limit Ac _{3 of the} heating temperature is the lowest temperature for utilizing transformation strengthening during quenching. Ar ₁ during heating
The reason for heating from a temperature below the point is to uniformly refine the structure and prevent deterioration of low temperature toughness. When reheating is performed from 500 ° C. or lower at which the transformation from austenite (γ) to ferrite (α) is completed, the γ structure after α to γ transformation is uniformly refined.

It is desirable that the steel pipe is quenched at a cooling rate of 10 ° C./sec or more. This is due to transformation strengthening due to formation of bainite structure, refinement of structure and coarse Cu during cooling.
This is to suppress precipitation. When Cu precipitates during cooling, the amount of precipitation hardening after aging treatment decreases, and high strength cannot be obtained. In order to increase the strength by Cu precipitation hardening (precipitation hardening by ε-Cu), aging treatment must be performed at an appropriate temperature. If the aging treatment temperature is less than 450 ° C., Cu precipitation is insufficient and high strength cannot be obtained, and if the aging treatment temperature exceeds Ac ₁ point, the Cu precipitates become coarse and the precipitation hardening ability is lost.

The steel pipe manufactured by roll forming includes a steel pipe for UOE forming and welding a rolled steel plate, a steel pipe for electric resistance welding of a rolled steel plate, and a steel pipe directly formed in the rolling process. Next, the reasons for limiting the constituent elements will be described. C
The lower limit of 0.02% is the minimum amount for ensuring the strength of the base material and the welded portion, the low temperature toughness, the precipitation hardening by the addition of Nb and V, and the effect of refining the crystal grains. However, if the C content is too large, the low temperature toughness, the field weldability and the sour resistance are significantly deteriorated, so the upper limit was made 0.09%.

Si is an element added for deoxidation and strength improvement, but if added in a large amount, it deteriorates the field weldability and HAZ toughness, so the upper limit was made 0.6%. Only Ti or Al is sufficient for deoxidizing steel, and Si is not necessarily added. Mn is an essential element for ensuring strength and low temperature toughness, and its lower limit is 1.3%.
However, if Mn is too much, not only the hardenability of the steel increases and the field weldability and HAZ toughness deteriorate, but also the center segregation of the continuously cast steel slab is promoted, and sour resistance and low temperature toughness also deteriorate. , The upper limit was 2.0%.

The purpose of adding Ni and Cu is to improve the strength of the low C steel of the present invention without deteriorating the low temperature toughness and sour resistance. Ni and Cu additions are Mn and C
It was found that compared with the addition of r and Mo, a hardened structure detrimental to low temperature toughness and sour resistance is less likely to be formed in the rolled structure (especially the central segregation zone of the slab), and the strength is increased. Cu addition increases strength mainly by Cu precipitation hardening. Therefore, the added amount of Cu must be at least 0.9%. However, if a large amount is added, the field weldability and HAZ toughness are deteriorated, so the upper limit was made 1.2%.
Ni is added to prevent Cu cracks during continuous casting and hot rolling, and the lower limit is 0.3%. However, if Ni is added in excess of 1.2%, it is not preferable for field weldability and the like, so the upper limit was made 1.2%.

In the steel of the present invention, N is an essential element.
b: 0.01 to 0.06%, Ti: 0.005 to 0.0
Contains 3%. Nb contributes to refinement of crystal grains and precipitation hardening in controlled rolling, and has an effect of strengthening steel.
However, if Nb is added in excess of 0.06%, the field weldability and HAZ toughness are adversely affected, so the upper limit is 0.0
It was 6%. Also, addition of Ti forms fine TiN,
When the slab is reheated and the austenite grains of the welded HAZ are suppressed from coarsening, the microstructure is refined, and the base metal and H
Improves low temperature toughness of AZ. In order to bring out such an effect of TiN, it is necessary to add at least 0.005% Ti. However, if the amount of Ti is too large, coarsening of TiN and precipitation hardening due to TiC occur and the low temperature toughness deteriorates, so the upper limit must be limited to 0.03%.

Al is an element usually contained in steel as a deoxidizing agent, and has an effect on the refinement of the structure. However, A
When the amount of l exceeds 0.06%, Al-based nonmetallic inclusions increase and impair the cleanliness of steel, so the upper limit was made 0.06%. Deoxidation is also possible with Ti or Si, and Al does not necessarily have to be added. Furthermore, in the present invention, the amounts of P and S that are impurity elements are 0.015% or less,
It is made 0.0010% or less. The main reason is to improve sour resistance and further improve low temperature toughness of the base material and HAZ toughness. Reduction of the amount of P reduces center segregation of the continuously cast slab, improves sour resistance, prevents intergranular fracture, and improves low temperature toughness. Further, the reduction of the amount of S has the effect of reducing the stretched MnS and improving the sour resistance and the ductility and toughness.

N forms TiN and suppresses coarsening of austenite grains in the slab reheating and in the welded HAZ and improves the low temperature toughness of the base metal and HAZ. The minimum amount required for this is 0.001%. However, if the amount of N is too large, it may cause a flaw in the surface of the slab or deterioration of the HAZ toughness due to solid solution N, so the upper limit must be suppressed to 0.006%.

Next, the reason for adding Ca, V and Cr will be explained. The main purpose of adding these elements to the basic composition is to increase the manufacturable plate thickness and improve the properties such as the strength and toughness of the base material without impairing the excellent characteristics of the steel of the present invention. Is. Therefore, the amount added is of a nature that should be limited by itself.

Ca controls the morphology of sulfide (MnS),
In addition to improving low temperature toughness (increasing absorbed energy in the Charpy test, etc.), it also exerts a remarkable effect in improving sour resistance. In particular, in the steel of the present invention utilizing the separation in the impact test, the absorbed energy in the Charpy test and the like tends to decrease, so that the addition of Ca is essential. However, if the amount of Ca is less than 0.001%, there is no practical effect, and if it exceeds 0.005%, Ca
O-CaS is produced in a large amount to form clusters and large inclusions, which not only impairs the cleanliness of steel, but also adversely affects on-site weldability. Therefore, the amount of Ca added is 0.001
It was limited to 0.005%.

V has almost the same effect as Nb, but its effect is much weaker than that of Nb. The upper limit is 0.10% in terms of field weldability and HAZ toughness.
Cr increases the strength of the base material and the welded portion, but if it is too large, the local weldability and HAZ toughness are significantly deteriorated. Therefore, the upper limit of the amount of Cr is 0.5%. The lower limits of 0.01% and 0.1% of the amounts of V and Cr are the minimum amounts in which the effect on the material due to the addition of the respective elements becomes remarkable.

[0021]

EXAMPLES Examples of the present invention will be described. Steel pipes were manufactured by various manufacturing methods from billets of various steel components by a converter-continuous casting method. These were heat-treated under various conditions and various properties were investigated. The mechanical properties were investigated in the direction perpendicular to the axial direction of the steel pipe. The sour resistance of the steel pipe containing Ca was investigated. Sour resistance is BP solution (hydrogen sulfide saturated artificial seawater,
After immersion in a pH of 4.8 to 5.4) for 96 hours, ultrasonic flaw detection was performed on the surface of the test piece, and the crack area ratio (%) of the test piece was evaluated.

Examples are shown in Tables 1 and 2 (continued from Table 1-
1) and Table 3 (continued from Table 1-2).

[0023]

[Table 1]

[0024]

[Table 2]

[0025]

[Table 3]

The steel pipe produced according to the present invention has excellent strength, low temperature toughness and sour resistance. On the other hand, the comparative steels are not suitable in terms of chemical composition or heat treatment conditions for steel pipes, and either characteristic is inferior. Steel 9 has too much C, so low temperature toughness (Charpy absorbed energy, transition temperature), HIC resistance
Inferior in nature. Steel 10 has a small amount of Mo added and an excessively large amount of Mn, so that the Charpy absorbed energy is low and the H resistance is high.
The IC property is bad. Steel 11 does not contain Nb, so
Its strength is slightly lower than that of Nb-added steel, and its Charpy transition temperature is high (balance between strength and low temperature toughness is poor). Steel 12 is T
Since i is not added, the Charpy transition temperature is high and the HIC resistance is poor. Steel 13 does not achieve the target strength because the amount of Cu added is too small. Since the steel 14 has too little Ni content, the mechanical properties are reasonable, but many minute flaws occur on the surface of the steel pipe and cannot be used as a line pipe. Steel 15 has an appropriate chemical composition, but has a high Charpy transition temperature because the heating start temperature in the manufacturing conditions is too high. Since the heating temperature of steel 16 is too low, the strength is low and the low temperature toughness is poor. Steel 17 has a low temperature toughness because the heating temperature is too high. Since the tempering temperature of steel 18 is too low, the effect of Cu precipitation effect is not exhibited and the strength is low. Steel 19
Since the tempering temperature is too high, the Cu precipitates are coarsened, the precipitation hardening ability is lost, and the strength is low.

[0027]

Industrial Applicability According to the present invention, it has become possible to stably manufacture an ultra-high-strength line pipe (API standard X100 or more) excellent in low temperature toughness, field weldability and sour resistance.
As a result, the safety of the pipeline was significantly improved, and the construction efficiency and transportation efficiency of the pipeline were dramatically improved.

Claims (2)

[Claims] 1. By weight%, C: 0.02 to 0.09%, Si: 0.6% or less, Mn: 1.3 to 2.0%, P: 0.015% or less, S: 0. 0.0010% or less, Ni: 0.3 to 1.2%, Cu: 0.9 to 1.2%, Mo: 0.1 to 0.5%, Nb: 0.01 to 0.06%, Ti : 0.005 to 0.03%, Al: 0.06% or less, N: 0.001 to 0.006%, and the balance is iron and inevitable impurities. The steel pipe manufactured as described above is heated by induction heating from a temperature of 500 ° C. or lower to a temperature range of Ac ₃ point to 1000 ° C. for quenching treatment, and then tempered in a temperature range of 450 ° C. to Ac ₁ point. A method for producing a high-strength steel pipe having excellent low temperature toughness, which is characterized by the following. 2. The starting steel billet further has Ca: 0.001 to 0.001.
0.005%, V: 0.01 to 0.10%, Cr: 0.
The method for producing a high-strength steel pipe having excellent low-temperature toughness according to claim 1, characterized in that it contains 1 to 0.5% of one kind or two or more kinds.